Tachometers are known based on the principle of varying a magnetic field as a function of the speed of rotation of a rotating member and of using a coil to transform these field variations into an electrical current of variable frequency equal to the frequency at which the field varies. Thus, an electrical signal is generated representative of the speed of rotation of the rotating member.
One embodiment of this type of device thus comprises a stationary permanent magnet having one of its poles in the form of a disk or ring with teeth around its periphery and received in a tubular body likewise having a toothed surface facing the toothed surface of the magnet and constituting a part for closing the magnetic circuit and extending to the other pole.
It will be understood that the teeth constitute a means for varying the air gap between the pole in question and the tubular part. If the tubular part is rotated, then its teeth are moved relative to the teeth on the pole of the magnet, thereby changing the air gap and consequently changing the magnetic field. Assuming that the teeth are at the same pitch on the tubular part and on the magnet, a relative angular displacement equal to one pitch unit causes the air gap to vary through a maximum and a minimum. The magnetic field varies in the same way and the current induced in a winding through which the variable magnetic field passes will give rise to a signal representative of the speed of rotation of the tubular part.
A similar device can therefore be used as a tachometer or speed sensor for a body rotating at a variable speed. When applied to an aircraft wheel, for example, the tubular part is coupled to the wheel and the fixed part is integral with the axle. However, it has been observed that this sensor is sensitive to the vibration to which it is subjected, and that the tubular body may enter into resonance with such vibration. The vibrational deformation of the tubular body (analogous to that of a bell) give rise to changes in the air gap which constitute a source of an interfering electrical signal. The frequency of this noise, and sometimes its amplitude, are of the same order as the corresponding features of the signal at low speeds of rotation of an aircraft wheel. In the past, this "noise" has been removed by filtering, thereby reducing the useful range of the sensor.
The present invention seeks to provide a different solution for removing this noise, whereby sensor operation is maintained over a wider frequency band.